1. Field of the Invention
This invention relates to a sintered silicon nitride and a production method thereof, which sintered nitride has a high temperature strength and suitable for the material of fine ceramic structural parts which are usable throughout a wide industrial field such as automobiles, mechanical apparatuses, chemical apparatuses and aeronautical and astronautical apparatuses.
2. Description of the Prior Art
Sintered silicon nitrides whose main component is silicon nitride are chemically stable and high in mechanical strength at ordinary and high temperatures, and therefore are suitable for the material of sliding parts such as bearings and engine parts such as rotors of turbochargers. Since silicon nitride is difficult to be singly sintered, it is usually sintered upon adding a large amount of sintering assistants such as MgO, Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3. Such sintering method is disclosed, for example, in Japanese Patent Provisional Publication Nos. 49-63710, 54-15916 and 60-137873.
However, in the conventional sintered silicon nitride produced by adding a large amount of the sintering assistants such as MgO, Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3, there exists a glass phase having a low melting point in grain boundary in the sintered silicon nitride. Accordingly, structural parts formed of this sintered silicon nitride are low in high temperature performance such as creep resistance, high temperature strength and oxidation resistance.